1. Field of the Invention
The present invention relates to protection structures of Shockley diode type. It more specifically aims at forming such structures with low turn-on voltages.
2. Discussion of the Related Art
FIG. 1 very schematically shows a bidirectional protection structure of double Shockley diode type, connected between terminals A1 and A2 of a circuit to be protected (not shown). This structure comprises two Shockley diodes 1 and 3 in antiparallel between terminals A1 and A2. Diode 1 is formed of a stack of four layers, respectively 1a to 1d, of alternating conductivity types. Layer 1a, of type P, is connected to terminal A1, and layer 1d, of type N, is connected to terminal A2. Diode 3 is formed of a stack of four layers, respectively 3a to 3d, of alternating conductivity types. Layer 3a, of type P, is connected to terminal A2, and layer 3d, of type N, is connected to terminal A1.
When a positive overvoltage occurs between terminals A1 and A2 and exceeds a threshold VBR, the NP blocking junction of diode 1, formed by regions 1a and 1c, becomes conductive by avalanche effect. Then, by thyristor effect, the voltage drop across diode 1 becomes much lower than the avalanche voltage. When a negative overvoltage occurs between terminals A1 and A2, the overvoltage is similarly removed via diode 3.
At the end of the overvoltage, when the current flowing through the Shockley drops below a threshold IH, the diode turns back off (thyristor opening).
Shockley diodes are generally used for protection involving high turn-on thresholds. Indeed, Shockley diodes with low turn-on thresholds, for example, VBR<25 V can be difficult and expensive to manufacture. Further, it is generally difficult to obtain both a low turn-on voltage VBR and an optimum turn-off threshold IH.
It would be desirable to be able to more easily manufacture Shockley diodes with low turn-on thresholds.